Positive-acting lower power thermally-responsive bimetallic switch



May 27, 1969 A. JENSEN 3, POSITIVEACTING LOW POWER THERMALLY'RESPONSIVEBIM LLIC SWITCH Filed D80. 6, 1966 United States Patent U.S. Cl. 337-1 5Claims ABSTRACT OF THE DISCLOSURE A compact, sensitive and positiveacting thermally-responsive switch having bimetallic contacts normallyclosed. A flexible resistive film supported on a flexible insulatingstrip, is positioned between the two bimetallic strip contacts tomaintain the open position of the bimetallic contacts, and preventintermittent closing thereof, with the application of low power.

This invention relates generally to thermally-responsive switches andmore particularly to switches using bimetallic contacts.

Thermally-responsive switches having bimetallic strip contacts are knownin which a heating resistor, for example a heating coil, is situatednext to or envelopes the bimetallic strips and maintains the switchopen, after it has once opened, by application of heat. The knownthermally-responsive switches, however, require a relatively large powerinput to be applied thereto since a relatively large amount of heat, andtherefore power or potential and current, for maintaining the bimetalliccontacts open reliably and positively. In these switches the heatingresistor is short circuited as long as the switch is closed. Heating ofthe heating resistor begins when the switch opens. If an appropriatepower source cannot be provided to enable the heating resistor to reacha high temperature quickly the bimetallic strip contacts may coolsufiiciently after having opened the switch so that they tend to move ina direction of closing the switch and even close it and cause it tooperate intermittently.

The switch in accordance with the invention overcomes the defect ofintermittent operation or chattering of switches of this type andprovides for a switch held effectively in an open position with very lowpower input requirements.

A feature of the thermally operated switch construction in accordancewith the invention is the provision of a no lag flexible resistive filmsupported on a flexible insulating strip and electrically connectedbetween the bimetallic strip contacts of the switch adjacent the freeends of the strip contacts, with a free loop portion allowing theresistive film to extend or assume a draped condition when the contactsare in an open and a closed condition respectively. The resistive filmhas opposite ends connected to the respective bimetallic strips andmaking intimate electrical connection therewith. The resistive film hasa resistance, which may vary along the length thereof, effective togenerate heat under a low application of power. The heat generated issufficient to keep contact portions of the bimetallic strip contactsspaced apart once the bimetallic strips have deflected due to theheating thereof in response to application of a potential differencebetween them. The film resistor serves only to quickly make up for heatlosses in the bimetallic strips once they have assumed a spaced-apartposition due to their bending in response to internal heat generated bycurrent flow therethrough.

While it is known to use, on bimetallic strip contacts,

3,447,] 13 Patented May 27, 1969 an insulating layer and a resistivefilm on the metal layers having the greatest coefiicient of expansion inpractice, however, difficulties have been encountered in connecting theresistive film with appropriate leads. The problem has been that atleast one terminal or end portion of the film must move with thebimetallic strips as they deflect or bend. The difficulties heretoforeencountered are avoided in the present invention because the ends of theresistive film which correspond to the freely moving ends of thebimetallic strips are connected directly to the bimetallic strips. Theintermediate portion of the resistive film forms a loop free of thecontacts, near the fixed points of the bimetallic strip contacts. Thisloop portion is free to extend and allow unimpaired deflection of thebimetallic strips.

The invention is partciularly applicable to bimetallic strip switches inwhich the switches are normally closed. The construction of the newswitch permits very stiff, and small switch assemblies to be constructedwhich react rapidly due to the large effective length of the bimetallicstrips and which are positive in their opening and closing. The switchin accordance with the invention is more positive and sensitive andpermits more compact construction than heretofore.

Another feature of the switch in accordance with the invention is theprovision of single insulating flexible strip support for the resistivefilm which is prefabricated and is simply bonded to the bimetal stripsthus mounting the resistive film automatically on the bimetallic strips.

In order to avoid heat build-up or burn-through on the insulative stripsection forming the loop between the bimetal strips the conductance ofthe resistive film can be made high compared with the conductance of theresistive film portions disposed adjacent the bimetal strips physicallypositioned parallel thereto. Provision is made to vary the conductivityin this portion by providing a current path as a film cover over theloop portion of the resistive film.

Other features and advantages of the switch in accordance with thepresent invention will be better understood as described in thefollowing specification and appended claims, in conjunction with thefollowing drawings in which:

FIG. 1 is a fragmentary view, partly in section, of bimetallic switchcontacts for a switch to be provided with the invention;

FIG. 2 is an enlarged cross section view of a portion of the switchparts illustrated in FIG. 1; and

FIG. 3 is a schematic diagram of a motor provided with a starter switchembodying the invention.

While the switch in accordance with the invention is suitable as astarting switch, and will be so described, for single-phase split-phasemotors in which the switch is connected to an auxiliary winding so thatthe starting current is reduced when the motor is up to speed. Thoseskilled in the art will understand that the principles are equallyapplicable to time-delay switches and the like.

As illustrated in FIG. 1 an insulator 1 supports bimetallic stripcontacts 2, 3. The bimetallic strips 2, 3 are made of dissimilar metalsand have the metallic layers thereof having the greater temperaturecoefficient of expansion confronting each other and are provided at thefree ends thereof with contacts or contact portions 4, 5 normallyclosed. The bimetallic strips are provided with end portions 6, 7 fixedin the support 1 and to which electrical connection is made to a switchmade in accordance with the invention as later described. When apotential difference is applied to the bimetallic strip contacts thecontact portions 4, -5 will assume a spaced apart or open position sincethe bimetallic strips will bend or deflect outwardly or away from eachother in response to the heat generated therein in response to currentflowing therethrough.

Provision is made, in a switch construction in accordance with theinvention, for keeping the contact portions 4-, 5 spaced apart once theswitch is open by a quickresponse element in the form of a resistivelayer which replenishes heat losses in the bimetallic strips so thatthey remain in their bent or deflected position thereby keeping thecontact portions 4, 5 open. Thus a flexible resistive film 8 is providedand supported on an insulating strip 9 having its opposite end portions10, 11 disposed in intimate contact with the layers of the bimetallicstrips having the greater temperature coefficient of expansion. A middleor intermediate length or portion v12; of the resistive film forms aloop between the two bimetallic strips 2, 3 which is free of thebimetallic strips and will allow suflicient and unimpeded deflection orbending of the bimetallic strips to assume a spaced apart position whenthe switch opens.

The resistive film 8 is electrically connected to the bimetallic strips2, 3 at connections 13, 14 in the neighborhood of the contact portions4, 5 and the strip 9 is provided with an adhesive layer 15 only in theregion of the portions 10, 11 bonded and held in intimate contact withthe bimetallic strips. The center section 12 of the resistive film 8 isprovided with a metal layer 16 overlying the film as illustrated in FIG.2, which fragmentarily illustrates substantially one half of theinsulating strip and resistive layer thereon. The metal layer 16provides a different conductivity value in the intermediate length andavoids excessive heating of the loop or tree portion of the resistivefilm.

One example of a construction of a switch in accordance with theinvention has been constructed in which polytetraflurethylene is usedfor the insulating strip 9. The resistive film 'formed on the supportstrip 9 consists of a mixture of epoxy resin and collodial graphite. Themixture is made so that the film has the desired resistancecharacteristics for the desired switch. Epoxy resin is used as the layer;15 for bonding the support strip onto the metallic plates or strips andthe connections 13, 14 are made of a mixture of an adhesive andcollodial silver. The connections consist of conductive material whichis disposed over the ends of the film and supporting strip effectingelectrical contact between the resistive film and the bimetallic stripsas illustrated in FIG. 1. If necessary, in view of the environment inwhich the switch will operate, the resistive film 8 can be coated withan insulating layer and a reflecting layer, not shown, in order to avoidheat losses.

FIG. 3 illustrates an application of a switch in accordance with theinvention as applied to a switch for motor starting. As illustrated, acircuit having connections 17, 18 for applying power from a source, notshown, to a single-phase asynchronous motor 19' having a main winding 20and a starting winding 21 energized when a switch 22 is closed. Astarting switch 23 built in accordance with the invention as heretoforedescribed, is connected in series with the starting winding. When themain switch 22 is closed current flow, determined by the windingresistances, flows through the main winding 20 as well as through theauxiliary winding 21. The auxiliary winding current heats the bimetallicstrips of the switch 23, which are in series with the starting winding,with a result that the switch opens. Upon opening of the switch theresistive film 8 has instantaneously applied to it the vector differencebetween the voltages of the main winding 20 and the auxiliary winding21. This voltage potential dif- :Eerence remains approximately constanteven with main supply fluctuations at the terminals 17, 1-8 and producessuflicient power to heat the starter switch sufficiently to maintain itin the open position due to the current flow in the resistive film ofthe starting switch 23.

Those skilled in the art will understand that the bi- 4 metallic switch23 in accordance with the invention has the current going through thebimetallic contacts while it is closed so that the resistive film isprotected from application of excess current. When the switch opens onlya low current flows through the resistive film which has only to restoreto, or compensate for, heat losses of the bimetallic contacts tomaintain the switch positively open. Since the film is immediatelyefiective there is no lag and no possibility of the contacts efiectingeven momentary closure. Moreover, the switch can be provided withsuitable adjusting screws, not shown, on one bimetallic strip forsetting the contact pressure between the contacts 4 and 5 so that thepositive action of the switch can be further maximized.

While a preferred embodiment of the invention has been shown anddescribed it will be understood that many modifications and changes canbe made within the true spirit and scope of the invention.

What -I claim and desire to be secured by Letters Patent is:

1. In a device for opening and closing an electric-a1 circuit having apair of contacts comprising a pair of electrically conductive bimetallicstrips connectable in series in said circuit, each strip comprisinglayers of dissimilar metals having different temperature coeflicients ofexpansion and contact portions at free ends thereon normally inelectrical contact and automatically operably to a spaced-apart,circuit-opening position in response to heating thereof upon applicationof a selected potential difference between said bimetallic strips, alag-free, quick response electrical means connected to and parallel withsaid bimetallic strips in said device and said circuit to continue toapply heat to said bimetallic strips substantially immediately upon saidbimetallic strips assuming said open-circuit position and heat effectiveto maintain said bimetallic strips heated and in said open-circuitposition, said quick-response electrical means comprising a flexibleresistive film, connection means connecting opposite ends of saidresistive film to said bimetallic strips, and said resistive film havingan intermediate loop portion free of the bimetallic strips.

2. In a device according to claim 1, in which said opposite ends of saidresistive film are connected to said bimetallic strips adjacent saidcontact portions, and each of said bimetallic strips having a metallayer thereof having the greater temperature coefficient of expansionconnected to respective ones of said connection means.

3. In a device according to claim 1, including a flexible strip ofinsulating material on which said resistive film is supported, saidstrip of insulating material having portions bonded to said bimetallicstrips and an intermediate loop portion supporting said loop portion ofsaid resistive film.

4. In a device according to claim '1, in which said resistive filmcomprises a film having different conductance along different lengthsthereof, thereby to preclude overheating and rupture of said film.

5. In a device according to claim 1, including a metallic layeroverlying said resistive film along a length corresponding to at least apart of said intermediate loop portion and said length thereby havinggreater conductivity than the remainder of said film.

References Cited UNITED STATES PATENTS 2,403,803 7/ 1946 Ke'arsley2l9-5l1 BERNARD A. GILHEANY, Primary Examiner.

R. L. COHRS, Assistant Examiner.

US. Cl. X.R.

